Problem: $ B = \left[\begin{array}{rrrr}1 & -2 & 1 & 2 \\ 1 & 4 & 2 & -1\end{array}\right]$ What are the dimensions of matrix $ B$ ?
Explanation: The first dimension is the number of rows in the matrix. How many rows does $ B$ have? The second dimension is the number of columns in the matrix. How many columns does $ B$ have? So, $ B$ is a 2 $\times$ 4 matrix.